Frit material for use with phosphor coating in fluorescent lamp

ABSTRACT

A frit material that improves the adhesion of phosphor coatings in fluorescent lamps has the general composition (Ca,Ba)O.B 2  O 3 .P 2  O 5 .

TECHNICAL FIELD

This invention relates to fluorescent lamps, particularly annular shapedfluorescent lamps, and to the use of a frit material to improve theadhesion of the phosphor coating to the lamp envelope.

BACKGROUND ART

Annular fluorescent lamps are generally manufactured by phosphor coatinga straight glass envelope, baking the coating, heating the coated glassenvelope to its softing temperature and then bending the coated envelopeto a torus shape. If an adhesive is not used with the phosphor coating,the coating can flake or peel off during the bending operation. Adhesivematerials that have been used in the past are alkali and alkaline earthborates, such as are disclosed in U.S. Pat. Nos. 2,905,572, 3,503,780and 4,121,132. Zinc phosphate has also been used, as disclosed in U.S.Pat. No. 3,963,639.

A problem with the use of borates as the adhesive in phosphor coating isthat it is difficult to remove all traces of water therefrom duringlehring. Residual water in the coating is detrimental to lamp operationsand lamp life. A problem with the use of zinc phosphate as the adhesiveis that it does not have adequate adhesive power to prevent flakingduring bending.

DISCLOSURE OF INVENTION

This invention discloses a frit material which can be added in thephosphor suspension and which improves the adherence of the phosphor tothe interior wall of the lamp envelope. The frit material softens duringlehring and improves the bonding of the phosphor particles to each otherand to the glass wall envelope. The adhesive is an amorphousborate-phosphate frit material having the general compositionCa_(1-x+2n) Ba_(x) O_(1+2n) ·mB₂ O₃ ·nP₂ O₅, where x=0.3±0.1, m=2±0.3and n=1±0.3. The frit material is added to an aqueous suspension ofphosphor particles which is milled to the desired consistency, and thencoated on the inner tubular envelope wall. The envelope is then airlehred to remove organic binder and is subsequently reheated and bentinto a torus shape. The borate-phosphate frit of this invention providesvery good adhesion of the phosphor and also provides good lampbrightness. The borate-phosphate frit fuses more readily than previousfrits and is especially advantageous in small diameter circularfluorescent lamps.

BRIEF DESCRIPTION OF THE DRAWING

The drawing shows an annular shaped fluorescent lamp, partly in section,incorporating the frit material of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

A borate-phosphate frit material according to this invention has thegeneral formulation Ca_(1-x+2n) Ba_(x) O_(1+2n) ·mB₂ O₃ ·nP₂ O₅, Theaddition of phosphate to the raw mixture results in a lower meltingpoint amorphous frit as compared to the crystalline frit disclosed in3,503,780 which does not contain phosphate. The borate-phosphate fritcan be considered as a eutectic melt of Ca_(1-x) Ba_(x) B₄ O₇ and Ca₂ P₂O₇, with the ratio of the latter to the former being approximately equalto n. The borate-phosphate frit has lost the crystalline structure andproperties of both the Ca_(1-x) Ba_(x) B₄ O₇ and Ca₂ P₂ O₇. The additionof the phosphate group has improved the bonding and adhesion of thephosphor particles to themselves as well as to the glass envelope,especially in the case of the commonly used halophosphate phosphors.

The frit material is generally prepared by (1) homogeneous mixing of rawmaterial in a blender, (2) firing of the intimate mixture in a furnace,(3) milling of cooled frit material to a fine powder, and (4) sievingthe powder through fine meshed screens. The raw material may beintroduced in various forms of compounds as in the art of phosphormanufacture; for example, calcium and barium may be introduced as theforms of hydroxide, carbonate, nitrate and sulfate; boron may beintroduced as boric acid; and phosphate may be introduced as calciumpyrophosphate, calcium hydrogen phosphate, and/or various forms ofammonium phosphate. However, best results were obtained when alkalineearth hydroxide, boric acid and calcium hydrogen phosphate were used. Asis known in the phosphor art, the purity of the ingredients isimportant; therefore, only reagent or better grade chemicals are used.The blending may be accomplished in a conventional twin shell blender orother blending device which gives homogeneous mixing. In order to obtainan intimate mixture, only fine size particles of less than 100 micronsare used. The solid state reaction is carried out by firing the intimatemixture in a furnace at a temperature from about 600° to 1000° C. toproduce the frit composite. The furnace atmosphere is not critical;therefore electrical or gas fired furnaces may be used. The firing orcalcine time may vary from 0.5 to 5 hours. The material may be regroundand refired if desired; however, one step firing was sufficient toproduce satisfactory frit material for lamp application. After thematerial has been removed from the furnace and cooled, it is milled andsieved. The frit must be in fine powder form to blend with phosphorpowder and to provide adequate bonding strength for lamp application.Either wet or dry milling can be used, and powder passing through a 200mesh screen was satisfactory for use in circular fluorescent lamps. Thefrit to phosphor ratio may vary from about 2 to 10% by weight; normally,4% is sufficient to provide enough adhesive power without sacrificingbrightness.

In one example, 51.86 grams Ca(OH)₂, 96.64 grams Ba(OH)₂ ·8H₂ O, 247.32grams H₃ Bo₃ and 240.3 grams CaHPO₄ were blended in a shaker untilhomogeneous and then loaded in a 73/4 inch high crucible. The cruciblewas covered and the mixture was then fired in a gas furnace at 800° C.for 3 hours. After firing and cooling, the mixture was ballmilled andsieved through 200 mesh screens.

The advantages of this frit over prior art frits is its superioradhesion of the phosphor to the glass and its lower fusion temperature.An important feature of the frit is its essentially completetransparency to visible light, 400 to 700 nanometers. Thus, use of thefrit does not substantially detract from lamp efficiency in that thefrit will not add adsorptive characteristics to the phosphor coating.

A fifty pound batch for a small scale production quantity could beprepared as follows. 4.1 lbs of calcium hydroxide, 7.5 lbs of bariumhydroxide octahydrate, 19.5 lbs of boric acid and 18.9 lbs of calciumhydrogen phosphate are throughly blended in a drum for one hour. The rawmaterial blend is then loaded into crucibles, for example, seven inchhigh silica crucibles, and covered. The crucibles are then fired in agas fired furnace for three hours at 800° C. The crucibles are thenremoved from the furnace and allowed to cool to room temperature, afterwhich the material is broken by suitable crushing or milling means, andthen sifted through a 100-mesh screen.

The screened product has the following typical chemical analysis: 25-28%by weight calcium; 5.2-7.5% barium; 6.0-7.1% boron; and 37-45%phosphate. X-ray analysis shows the material to have an amorphousstructure. Spectrophotometrically, the product is transparent in thevisible spectrum, 400 to 700nm. The softening point of the frit is lessthan 600° C.

The drawing shows a circular fluorescent lamp in which the frit of thisinvention can be used. The lamp comprises a circular glass envelope 1sealed at each end, as is well known. The lamp contains the usualelectrode 2 at each end. The electrodes are contacted to base pins 3 forenergizing the lamp. Disposed on the inner surface of envelope 1 is aphosphor coating 4 which contains a small amount of the borate-phosphatefrit of this invention. In one example for FC8T9/CW 22 watt circlinefluorescent lamps employing this invention, straight glass tubes of theproper length were internally coated with an aqueous suspension of coolwhite halophosphate phosphor containing 4% of borate-phosphate frit asper this invention. The amount of phosphor plus frit deposited in eachtube was about 2 grams. The coated tubes were then dried with hot air toevaporate the water. The coated tubes were then lehred at about 600° C.This lehring step drives off water soluble organic matter that was usedas a binder in the aqueous suspension. During this step, some of theborate-phosphate frit is melted, but not enough to complete the fusionprocess. The glass tubes are then heated to about 700° C., and are thenbent to the circline shape. This completes the fusion process of thefrit. The lamp is then completed in the usual manner.

A comparison was made of such FC8T9/CW lamps containing 4% of theborate-phosphate frit as per this invention in comparison to lampscontaining 4% of the glass frit that was used in the prior art. Theresults as as follows, in Table I.

                  TABLE I                                                         ______________________________________                                                                  100 hour      500 hour                              Phosphor                                                                              Lumens,  Lumens,  main-  Lumens,                                                                              main-                                 Coating 0 hours  100 hours                                                                              tenance                                                                              500 hours                                                                            tenance                               ______________________________________                                        Cool White                                                                    plus 4%                                                                       borate-                                                                       phosphate                                                                     frit    1142     1097     96.1%  1037   90.8%                                 Cool White                                                                    plus 4%                                                                       glass frit                                                                            1148     1067     92.9%   988   86.1%                                 ______________________________________                                    

It can be seen that the frit of this invention yielded improvedmaintenance both at 100 hours and at 500 hours. In addition, theadhesion of this frit was superior to that of the prior art glass frit.

What is claimed is:
 1. A low melting point amorphous borate-phosphatefrit material having the general composition Ca_(1-x+2n) Ba_(x) O_(1+2n)·mB₂ O₃ ·nP₂ O₅, wherein x is between about 0.2 to 0.4, n is betweenabout 0.7 to 1.3, and m is between about 1.7 to 2.3.
 2. An aqueoussuspension for depositing a phosphor coating on a lamp envelopecomprising finely divided phosphor particles dispersed in water; atemporary organic binder which can be essentially completely removedfrom a lamp envelope on which the phosphor coating has been depositedupon firing under appropriate conditions; and a minor amount of anamorphous borate-phosphate adhesive frit which will bond the phosphorparticles to each other and to the lamp envelope upon firing underappropriate conditions, wherein the amount of borate-phosphate fritequals about 2 to 10 weight percent of the phosphate and the frit hasthe composition Ca_(1-x+2n) Ba_(x) O_(1+2n) ·mB₂ O₃ ·nP₂ O₅, where x isbetween about 0.2 to 0.4, n is between about 0.7 to 1.3, and m isbetween about 1.7 to 2.3.